Is there an external battery charger accessory available? I couldn't find one on the iRobot site, and Amazon listed one, but looks like no stock for quite some time based on the reviews. I'd like to run my 400 series as often as possible, and having an extra battery charging while the unit is running would be helpful.

No, using a stock automotive battery charger to charge a Roomba battery is not a good idea. Look for a Roomba Rapid Charger instead. If you use a stock charger that does not monitor the built-in thermistor of the Roomba battery then be sure and stick around during charging to disconnect the battery if issues develop.

Nitrowing wrote:Is this because a car charger outputs too much amps?Sticking around for a couple of hours watching a battery, not knowing what to look for??! Ummm....

Roomba charge the battery around 1.3A and also monitor the battery temperature. Heat kills battery. Roomba will switch to trickle charging mode after full charge by monitor the dV (I think - correct me if I am wrong).

A car charger may use too much current and heat up the battery too much too fast.

There are Radio Control grade chargers that can charge a 4S Li-ION or 12 cell NIMH/NICAD pack from a 12V supply. You can get a cheap PC supply that puts out 12V and use it to run the charger. Total investment can approach $100 though. I have had good luck with this charger:

If your supply is only putting out 14.72V then you probably will not fully charge the pack. That may be why it is not getting warm. Once the pack gets to 14.72V no current will flow. Most NIMHs and NICADs tend to peak near 1.5V which is 18V per pack.

As Mike has mentioned, a fully charged Roomba battery that is working will display about 17 volts dc when checked with a meter. The stock Roomba power supply has 22 volts dc that is output to the charging jack.

You can hook up the power supply directly to the battery but you will need to monitor the battery both for voltage and temperature. That monitoring is normally done by the Roomba charging software. If you bypass the Roomba charging system then things become more complicated and dangerous.

vic7767 wrote:As Mike has mentioned, a fully charged Roomba battery that is working will display about 17 volts dc when checked with a meter. The stock Roomba power supply has 22 volts dc that is output to the charging jack.

You can hook up the power supply directly to the battery but you will need to monitor the battery both for voltage and temperature. That monitoring is normally done by the Roomba charging software. If you bypass the Roomba charging system then things become more complicated and dangerous.

OK So I need a 17~18Vdc source to charge the battery. I'll go to Maplins again tomorrow. Unless I use my Black&Decker charger.

**EDIT**B&D charger is a no-no, it's a 31V output for it's 18V battery

I've directly connected the Roomba PSU to the battery and it's reading 16.7V

Nudnik wrote:I wonder if a laptop power adapter would work. For example my HP adapter has 19 volt output,...

It is necessary to include the PSU's amperage (current) as part of such "wondering".

When substituting an alternate dc PSU to do Rommba 4XXX charging (i.e., charging current will pass through the robot's internal charging control circuit) it will be wise to emulate the characteristics of its Fast Charger, in the limit, as shown in this post:viewtopic.php?p=25609#p25609You will prefer the current delivery capability to roll off at less than 1.3 amperes. Of course the PSU may invoke current limiting at lesser current levels, and the battery might reward you with more cycles. Charging at currents less than 1.25A,NOM will mean the process requires more time to complete -- so there is a trade there.

If you substitute a 19V, 3A PSU for Roomba's PSU, you stand the chance of ruining the robot's U2 & U4 MOSFET charging switches, since they will be dissipating more than twice the watts during the early phase of high-rate charging that they would if the Fast Charger was being used. Also, the low 19V level may not fully charge the battery because some of that voltage is lost within the controller before it reaches the battery terminal (the applied voltage would be around 17V, which means the battery would finish high-rate in the 15 to 16V range, but extended trickle charging might raise that to a normal level).

mfortuna wrote:While monitoring the pack look for the voltage to peak and then drop. Also the pack will get warm when fully charged.

Mike, Nitrowing's charging setup will not permit monitoring (unless he interrupts charging periodically to measure terminal voltage w/o the PSU connected, then plots those V vs t data) for that voltage peak. His only hope is to fully depend on using the battery's thermistor resistance to estimate cells' temperature.